Receptor on the surface of activated T-cells: ACT-4

ABSTRACT

The invention provides purified ACT-4 receptor polypeptides, antibodies against these polypeptides and nucleic acids encoding ACT-4 receptor polypeptides. Also provided are methods of diagnosis and treatment using the same. ACT-4 receptors are preferentially expressed on the surface of activated CD4 +  T-cells. ACT-4 receptors are usually expressed at low levels on the surface of activated CD8 +  cells, and are usually substantially absent on resting T-cells, and on monocytes and B-cells (resting or activated). An exemplary ACT-4 receptor, termed ACT-4-h-1, has a signal sequence, an extracellular domain comprising three disulfide-bonded intrachain loops, a transmembrane domain, and an intracellular domain.

TECHNICAL FIELD

This invention relates generally to the isolation and characterizationof a cell-surface receptor, termed ACT-4, and antibodies thereto, andthe use of the antigen and antibodies for monitoring and/or modulatingimmune responses.

BACKGROUND OF THE INVENTION

Immune responses are largely mediated by a diverse collection ofperipheral blood cells termed leukocytes. The leukocytes includelymphocytes, granulocytes and monocytes. Granulocytes are furthersubdivided into neutrophils, eosinophils and basophils. Lymphocytes arefurther subdivided into T and B lymphocytes. T-lymphocytes originatefrom lymphocytic-committed stem cells of the embryo. Differentiationoccurs in the thymus and proceeds through prothymocyte, corticalthymocyte and medullary thymocyte intermediate stages, to producevarious types of mature T-cells. These subtypes include CD8⁺ T cells(also known as cytotoxic/suppressor T cells), which, when activated,have the capacity to lyse target cells, and CD4⁺ T cells (also known asT helper and T inducer cells), which, when activated, have the capacityto stimulate other immune system cell types.

Immune system responses are elicited in several differing situations.The most frequent response is as a desirable protection againstinfectious microorganisms. However, undesired immune response can occurfollowing transplantation of foreign tissue, or in an autoimmunedisease, in which one of a body's own antigens is the target for theimmune response. Immune responses can also be initiated in vitro bymitogens or antibodies against certain receptors. In each of thesesituations, an immune response is transduced from a stimulating eventvia a complex interaction of leukocytic cell types. However, theparticipating cell types and nature of the interaction between celltypes may vary for different stimulating events. For example, immuneresponses against invading bacteria are often transduced by formation ofcomplexes between an MHC Class II receptor and a bacterial antigen,which then activate CD4⁺ T-cells. By contrast, immune responses againstviral infections are principally transduced by formation of MHC ClassI/viral antigen complexes and subsequent activation of CD8⁺ cells.

Over recent years, many leukocyte cell surface antigens have beenidentified, some of which have been shown to have a role in signaltransduction. It has been found that signals may be transduced between acell-surface receptor and either a soluble ligand or acell-surface-bound ligand. The amino acid sequences of leukocyte surfacemolecules comprise a number of characteristic recurring sequences ormotifs. These motifs are predicted to be related in evolution, havesimilar folding patterns and mediate similar types of interactions. Anumber of superfamilies, including the immunoglobulin and nerve growthfactor receptor superfamilies, have been described. Members of the nervegrowth factor receptor family include NGFR, found on neural cells; theB-cell antigen CD40; the rat OX-40 antigen, found on activated CD4⁺cells (Mallet et al., EMBO J. 9:1063-1068 (1990) (hereby incorporated byreference for all purposes); two receptors for tumor necrosis factor(TNF), LTNFR-1 and TNFR-II, found on a variety of cell types; 4-1BBfound on T-cells; SFV-T2, an open reading frame in Shope fibroma virus;and possibly fas, CD27 and CD30. See generally Mallet & Barclay,Immunology Today 12:220-222 (1990) (hereby incorporated by reference forall purposes).

The identification of cell-surface receptors has suggested new agentsfor suppressing undesirable immune responses such as transplantrejection, autoimmune disease and inflammation. Agents, particularlyantibodies, that block receptors of immune cells from binding to solublemolecules or cell-bound receptors can impair immune responses. Ideally,an agent should block only undesired immune responses (e.g., transplantrejection) while leaving a residual capacity to effect desirableresponses (e.g., responsive to pathogenic microorganisms). Theimmunosuppressive action of some agents, for example, antibodies againstthe CD3 receptor and the IL-2 receptor have already been tested inclinical trials. Although some trials have shown encouraging results,significant problems remain. First, a patient may develop an immuneresponse toward the blocking agent preventing continuedimmunosuppressive effects unless different agents are available. Second,cells expressing the target antigen may be able to adapt to the presenceof the blocking agent by ceasing to express the antigen, while retainingimmune functions. In this situation, continued treatment with a singleimmunosuppressive agent is ineffective. Third, many targets fortherapeutic agents are located on more than one leukocyte subtype, withthe result that it is generally not possible to selectively block oreliminate the response of only specific cellular subtypes and therebyleave unimpaired a residual immune capacity for combating infectiousmicroorganisms.

Based on the foregoing it is apparent that a need exists for additionaland improved agents capable of suppressing immune responses,particularly agents capable of selective suppression. The presentinvention fulfills these and other needs, in part, by providing acellular receptor localized on activated human CD4⁺ T-lymphocytes.

SUMMARY OF THE INVENTION

In one embodiment of the invention, purified ACT-4 receptor polypeptidesare provided. The amino acid sequence of one such polypeptide, termedACT-4-h-1, is shown in FIG. 5. ACT-4 receptor polypeptides typicallyexhibit at least 80% amino acid sequence identity to the ACT-4-h-1 aminoacid sequence. The polypeptides usually comprise at least one, andsometimes all of the following domains: a signal sequence, anintracellular domain, a transmembrane domain and an extracellulardomain. Many polypeptides are characterized by their presence onactivated CD4⁺ T-cells and their substantial absence on resting T-cells.Some full-length polypeptides have a molecular weight of about 50 kDabefore deglycosylation and about 27 kDa thereafter.

The invention also provides extracellular domains of ACT-4 receptorpolypeptides. The extracellular domains typically comprise at least onedisulfide-bonded loop and sometimes three such loops. The extracellulardomains are usually soluble and capable of specific binding to an ACT-4ligand. Sometimes an extracellular domain is fused to a secondpolypeptide such as a constant region of an immunoglobulin heavy chain.Some extracellular domains consist essentially of an epitopespecifically bound by an antibody designated L106.

In another aspect of the invention, antibodies that specifically bind toan ACT-4-h-1 receptor polypeptide are provided. The antibodies areusually monoclonal antibodies. One example of such an antibody isdesignated L106. Some antibodies inhibit activation of CD4⁺ T-cells,whereas other antibodies stimulate activation of these cells. Someantibodies of the invention compete with the L106 antibody for specificbinding to an ACT-4-h-1 receptor polypeptide, and most of theseantibodies also compete with L106 for specific binding to activated CD4⁺T-cells. Other antibodies of the invention specifically bind to adifferent epitope than that bound by the L106 antibody. Also providedare fragments of the L106 antibody that specifically bind to anACT-4-h-1 receptor polypeptide.

Also provided are humanized antibodies comprising a humanized heavychain and a humanized light chain. The humanized light chain comprisesthree complementarity determining regions (CDR1, CDR2 and CDR3) havingamino acid sequences from the corresponding complementarity determiningregions of a L106 antibody light chain, and having a variable regionframework sequence substantially identical to a human light chainvariable region framework sequence. The humanized heavy chain comprisesthree complementarity determining regions (CDR1, CDR2 and CDR3) havingamino acid sequences from the corresponding complementarity determiningregions of an L106 antibody heavy chain, and having a variable regionframework sequence substantially identical to a human heavy chainvariable region framework sequence. The humanized antibodiesspecifically bind to an ACT-4-h-1 receptor polypeptide with a bindingaffinity that is within three-fold of the binding affinity of the L106antibody.

In another aspect, the invention provides nucleic acids fragmentsencoding the ACT-4 receptor polypeptides discussed supra. An example ofsuch a nucleic acid fragment comprises the nucleotide sequence encodingthe ACT-4-h-1 receptor shown in FIG. 5. The nucleic acid fragmentstypically exhibit at least eighty percent sequence identity to thenucleic acid sequence of FIG. 5.

The invention also provides isolated cell lines containing the nucleicacid fragments discussed supra. The cell lines usually express an ACT-4receptor polypeptide on their cell surface. Some of the cell lines arestable, as when the nucleic acid fragment is incorporated in the genomeof the cell line.

The invention also provides methods of screening for immunosuppressiveagents. An ACT-4-h-1 receptor polypeptide is contacted with a potentialimmunosuppressive agent. Specific binding between the ACT-4-h-1 receptorpolypeptide or fragment and the agent is then detected. The existence ofspecific binding is indicative of immunosuppressive activity.

The invention also provides methods of screening for an ACT-4 ligand. Abiological sample containing the ACT-4 ligand is contacted with anACT-4-h-1 receptor polypeptide. A complex is formed between the ligandand the ACT-4-h-1 receptor polypeptide. The complex is then dissociatedto obtain the ligand.

In another aspect, the invention provides methods of suppressing animmune response in a patient suffering from an immune disease orcondition. A therapeutically effective dose of a pharmaceuticalcomposition is administered to the patient. The pharmaceuticalcomposition comprises a pharmaceutically active carrier and a monoclonalantibody that specifically binds to an ACT-4-h-1 receptor polypeptide.

Also provided are methods of detecting activated CD4⁺ T-cells. A tissuesample from a patient is contacted with a monoclonal antibody thatspecifically binds to an ACT-4-h-1 receptor polypeptide. Specificbinding between the monoclonal antibody and the tissue sample isdetected. The existence of specific binding reveals the presence ofactivated CD4⁺ T-cells. The presence of activated CD4⁺ T-cells is oftendiagnostic of a disease or condition of the immune system.

Also provided are methods of inducing an immune response to a selectedantigen. A monoclonal antibody that specifically binds to an ACT-4-h-1receptor polypeptide and that stimulates activation of CD4⁺ T-cells isadministered to a patient. The patient is exposed to the selectedantigen.

The invention also provides ACT-4 ligands that specifically bind to anACT-4-h-1 receptor polypeptide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Two-color staining of peripheral blood lymphocytes to analyzeexpression of ACT-4-h-1 on different cell types.

FIG. 2: Kinetics of ACT-4-h-1 expression on alloantigen-activated CD4⁺T-cells. MCF=Mean channel fluorescence.

FIG. 3: Kinetics of ACT-4-h-1 expression on tetanus-toxoid-activatedCD4+ T-cells.

FIG. 4: Kinetics of ACT-4-h-1 expression on PHA-activated CD4⁺ T-cells.

FIG. 5: cDNA (upper) and deduced amino acid sequence (lower) ofACT-4-h-1. The Figure indicates the locations of an N-terminal signalsequence, two possible signal cleavage sites (vertical arrows), twoglycosylation sites (gly), a transmembrane domain (TM), a stop codon anda poly-A signal sequence.

FIG. 6: Construction of expression vector for production of stabletransfectants expressing ACT-4-h-1.

FIG. 7: FACS™ analysis showing expression of ACT-4-h-1 on stabletransfectants of COS-7, Jurkat and SP2/O cell lines.

FIG. 8: Fusion of an ACT-4-h-1 extracellular domain with animmunoglobulin heavy chain constant region to form a recombinantglobulin.

FIG. 9: Schematic topographical representation of recombinant globulinformed from fusion of an ACT-4-h-1 extracellular domain with animmunoglobulin heavy chain constant region to form a recombinantglobulin.

DEFINITIONS

Abbreviations for the twenty naturally occurring amino acids followconventional usage (Immunology—A Synthesis, (E. S. Golub & D. R. Gren,eds., Sinauer Associates, Sunderland, MA, 2nd ed., 1991) (herebyincorporated by reference for all purposes). Stereoisomers (e.g.,D-amino acids) of the twenty conventional amino acids, unnatural aminoacids such as α,α-disubstituted amino acids, N-alkyl amino acids, lacticacid, and other unconventional amino acids may also be suitablecomponents for polypeptides of the present invention. Examples ofunconventional amino acids include: 4-hydroxyproline,γ-carboxyglutamate, ε-N,N,N-trimethyllysine, ε-N-acetyllysine,O-phosphoserine, N-acetylserine, N-formylmethionine, 3-methylhistidine,5-hydroxylysine, ω-N-methylarginine, and other similar amino acids andimino acids (e.g., 4-hydroxyproline). In the polypeptide notation usedherein, the left-hand direction is the amino terminal direction and theright-hand direction is the carboxy-terminal direction, in accordancewith standard usage and convention. Similarly, unless specifiedotherwise, the lefthand end of single-stranded polynucleotide sequencesis the 5′ end; the lefthand direction of double-stranded polynucleotidesequences is referred to as the 5′ direction. The direction of 5′ to 3′addition of nascent RNA transcripts is referred to as the transcriptiondirection; sequence regions on the DNA strand having the same sequenceas the RNA and which are 5′ to the 5′ end of the RNA transcript arereferred to as “upstream sequences”; sequence regions on the DNA strandhaving the same sequence as the RNA and which are 3′ to the 3′ end ofthe RNA transcript are referred to as “downstream sequences”.

The phrase “polynucleotide sequence” refers to a single ordouble-stranded polymer of deoxyribonucleotide or ribonucleotide basesread from the 5′ to the 3′ end. It includes self-replicating plasmids,infectious polymers of DNA or RNA and non-functional DNA or RNA.

The following terms are used to describe the sequence relationshipsbetween two or more polynucleotides: “reference sequence”, “comparisonwindow”, “sequence identity”, “percentage of sequence identity”, and“substantial identity”. A “reference sequence” is a defined sequenceused as a basis for a sequence comparison; a reference sequence may be asubset of a larger sequence, for example, as a segment of a full-lengthcDNA or gene sequence given in a sequence listing, such as apolynucleotide sequence shown in FIG. 5, or may comprise a complete cDNAor gene sequence. Generally, a reference sequence is at least 20nucleotides in length, frequently at least 25 nucleotides in length, andoften at least 50 nucleotides in length. Since two polynucleotides mayeach (1) comprise a sequence (i.e., a portion of the completepolynucleotide sequence) that is similar between the twopolynucleotides, and (2) may further comprise a sequence that isdivergent between the two polynucleotides, sequence comparisons betweentwo (or more) polynucleotides are typically performed by comparingsequences of the two polynucleotides over a “comparison window” toidentify and compare local regions of sequence similarity. A “comparisonwindow”, as used herein, refers to a conceptual segment of at least 20contiguous nucleotide positions wherein a polynucleotide sequence may becompared to a reference sequence of at least 20 contiguous nucleotidesand wherein the portion of the polynucleotide sequence in the comparisonwindow may comprise additions or deletions (i.e., gaps) of 20 percent orless as compared to the reference sequence (which does not compriseadditions or deletions) for optimal alignment of the two sequences.Optimal alignment of sequences for aligning a comparison window may beconducted by the local homology algorithm of Smith & Waterman, Appl.Math. 2:482 (1981), by the homology alignment algorithm of Needleman &Wunsch, J. Mol. Biol. 48:443 (1970), by the search for similarity methodof Pearson & Lipman, Proc. Natl. Acad. Sci. (USA) 85:2444 (1988), bycomputerized implementations of these algorithms (FASTDB(Intelligenetics), BLAST (National Center for Biomedical Information) orGAP, BESTFIT, FASTA, and TFASTA (Wisconsin Genetics Software PackageRelease 7.0, Genetics Computer Group, 575 Science Dr., Madison, Wis.)),or by inspection, and the best alignment (i.e., resulting in the highestpercentage of sequence similarity over the comparison window) generatedby the various methods is selected. The term “sequence identity” meansthat two polynucleotide sequences are identical (i.e., on anucleotide-by-nucleotide basis) over the window of comparison. The term“percentage of sequence identity” is calculated by comparing twooptimally aligned sequences over the window of comparison, determiningthe number of positions at which the identical nucleic acid base (e.g.,A, T, C, G, U, or I) occurs in both sequences to yield the number ofmatched positions, dividing the number of matched positions by the totalnumber of positions in the window of comparison (i.e., the window size),and multiplying the result by 100 to yield the percentage of sequenceidentity. The terms “substantial identity” as used herein denotes acharacteristic of a polynucleotide sequence, wherein the polynucleotidecomprises a sequence that has at least 70, 80 or 85 percent sequenceidentity, preferably at least 90 to 95 percent sequence identity, moreusually at least 99 percent sequence identity as compared to a referencesequence over a comparison window of at least 20 nucleotide positions,frequently over a window of at least 25-50 nucleotides, wherein thepercentage of sequence identity is calculated by comparing the referencesequence to the polynucleotide sequence which may include deletions oradditions which total 20 percent or less of the reference sequence overthe window of comparison. The reference sequence may be a subset of alarger sequence, for example, as a segment of the full-length ACT-4-h-1sequence shown in FIG. 5.

As applied to polypeptides, the term “substantial identity” means thattwo peptide sequences, when optimally aligned, such as by the programsBLAZE (Intelligenetics) GAP or BESTFIT using default gap weights, shareat least 70 percent or 80 percent sequence identity, preferably at least90 percent sequence identity, more preferably at least 95 percentsequence identity or more (e.g., 99 percent sequence identity).Preferably, residue positions which are not identical differ byconservative amino acid substitutions. Conservative amino acidsubstitutions refer to the interchangeability of residues having similarside chains. For example, a group of amino acids having aliphatic sidechains is glycine, alanine, valine, leucine, and isoleucine; a group ofamino acids having aliphatic-hydroxyl side chains is serine andthreonine; a group of amino acids having amide-containing side chains isasparagine and glutamine; a group of amino acids having aromatic sidechains is phenylalanine, tyrosine, and tryptophan; a group of aminoacids having basic side chains is lysine, arginine, and histidine; and agroup of amino acids having sulfur-containing side chains is cysteineand methionine. Preferred conservative amino acids substitution groupsare: valine-leucine-isoleucine, phenylalanine-tyrosine, lysine-arginine,alanine-valine, and asparagine-glutamine.

The term “substantially pure” means an object species is the predominantspecies present (i.e., on a molar basis it is more abundant than anyother individual species in the composition), and preferably asubstantially purified fraction is a composition wherein the objectspecies comprises at least about 50 percent (on a molar basis) of allmacromolecular species present. Generally, a substantially purecomposition will comprise more than about 80 to 90 percent of allmacromolecular species present in the composition. Most preferably, theobject species is purified to essential homogeneity (contaminant speciescannot be detected in the composition by conventional detection methods)wherein the composition consists essentially of a single macromolecularspecies.

The term “naturally-occurring” as used herein as applied to an objectrefers to the fact that an object can be found in nature. For example, apolypeptide or polynucleotide sequence that is present in an organism(including viruses) that can be isolated from a source in nature andwhich has not been intentionally modified by man in the laboratory isnaturally-occurring.

The term “epitope” includes any protein determinant capable of specificbinding to an immunoglobulin or T-cell receptor. Specific binding existswhen the dissociation constant for antibody binding to an antigen is ≦1μM, preferably ≦100 nM and most preferably ≦1 nM. Epitopic determinantsusually consist of chemically active surface groupings of molecules suchas amino acids or sugar side chains and usually have specific threedimensional structural characteristics, as well as specific chargecharacteristics.

The term “higher cognate variants” as used herein refers to a genesequence that is evolutionarily and functionally related between humansand higher mammalian species, such as primates, porcines and bovines.The term does not include gene sequences from rodents, such as rats.Thus, the cognate primate gene to the ACT-4-h-1 gene is the primate genewhich encodes an expressed protein which has the greatest degree ofsequence identity to the ACT-4-h-1 receptor protein and which exhibitsan expression pattern similar to that of the ACT-4-h-1 protein (i.e.,expressed on activated CD4⁺ cells).

The term “patient” includes human and veterinary subjects.

DETAILED DESCRIPTION

I. ACT-4 Receptor Polypeptides

According to one embodiment of the invention, receptors on the surfaceof activated CD4⁺ T-cells (referred to as ACT-4 receptors) and fragmentsthereof are provided. The term ACT-4 receptor polypeptide is usedgenerically to encompass full-length receptors and fragments thereof.The amino acid sequence of the first ACT-4 receptor to be characterized[hereinafter ACT-4-h-1] is shown in FIG. 5. The suffix -h designateshuman origin and the suffix -1 indicates that ACT-4-h-1 is the firstACT-4 receptor to be characterized. The term ACT-4 receptor refers notonly to the protein having the sequence shown in FIG. 5, but also toother proteins that represent allelic, nonallelic, and higher cognatevariants of ACT-4-h-1, and natural or induced mutants of any of these.Usually, ACT-4 receptor polypeptides will also show substantial sequenceidentity with the ACT-4-h-1 sequence. Typically, an ACT-4 receptorpolypeptide will contain at least 4 and more commonly 5, 6, 7, 10 or 20,50 or more contiguous amino acids from the ACT-4-h-1 sequence. It iswell known in the art that functional domains, such as binding domainsor epitopes can be formed from as few as four amino acids residues.

ACT-4 receptor polypeptides will typically exhibit substantial aminoacid sequence identity with the amino acid sequence of ACT-4-h-1, and beencoded by nucleotide sequences that exhibit substantial sequenceidentity with the nucleotide sequence encoding ACT-4-h-1 shown in FIG.5. The nucleotides encoding ACT-4 receptor proteins will also typicallyhybridize to the ACT-4-h-1 sequence under stringent conditions. However,these nucleotides will not usually hybridize under stringent conditionsto the nucleic acid encoding OX-40 receptor, as described by Mallet etal., EMBO J. 9:1063-68 (1990) (hereby incorporated by reference for allpurposes) (See particularly FIG. 2A of the Mallet et al. reference).Stringent conditions are sequence dependent and will be different indifferent circumstances. Generally, stringent conditions are selected tobe about 50° C. lower than the thermal melting point (Tm) for thespecific sequence at a defined ionic strength and pH. The Tm is thetemperature (under defined ionic strength and Ph) at which 50% of thetarget sequence hybridizes to a perfectly matched probe. Typically,stringent conditions will be those in which the salt concentration is atleast about 0.02 molar at Ph 7 and the temperature is at least about 60°C. As other factors may significantly affect the stringency ofhybridization, including, among others, base composition and size of thecomplementary strands, the presence of organic solvents and the extentof base mismatching, the combination of parameters is more importantthan the absolute measure of any one.

Usually, ACT-4 receptor polypeptides will share at least one antigenicdeterminant in common with ACT-4-h-1 but will not be specificallyreactive with antibodies against the rat OX-40 polypeptide. Theexistence of a common antigenic determinant is evidenced bycross-reactivity of the variant protein with any antibody preparedagainst ACT-4-h-1 (see Section IV). Cross-reactivity is often testedusing polyclonal sera against ACT-4-h-1, but can also be tested usingone or more monoclonal antibodies against ACT-4-h-1, such as theantibody designated L106.

Often ACT-4 receptor polypeptides will contain modified polypeptidebackbones. Modifications include chemical derivatizations ofpolypeptides, such as acetylations, carboxylations and the like. Theyalso include glycosylation modifications (N- and O-linked) andprocessing variants of a typical polypeptide. These processing stepsspecifically include enzymatic modifications, such as ubiquitinizationand phosphorylation. See, e.g., Hershko & Ciechanover, Ann. Rev. Bioch.51:335-364 (1982). The ACT-4-h-1 protein, for example, is heavilymodified in that the observed molecular weight is about 50 kDa, whereasthe predicted molecular weight based on amino acid sequence is only 27kDa. Two putative glycosylation sites have been identified in itsextracellular domain.

ACT-4 receptors likely share some or all of the topological featuresfound for ACT-4-h-1. The amino acid sequence for ACT-4-h-1 contains a 22or 24 amino acid putative N-terminal signal sequence. The 24 amino acidsequence is more probably based on the criteria of von Heijne, NucleicAcids Res. 14: 4683-4690 (1986) (incorporated by reference for allpurposes). The ACT-4-h-1 receptor contains a single additionalhydrophobic stretch of 27 amino acids spanning residues 213-240. Thehydrophobic stretch probably corresponds to a transmembrane domain andits existence is consistent with ACT-4-h-1 being a type I integralmembrane protein (i.e., having a single transmembrane domain with theN-terminal domain comprising the extracellular region and the C-terminuscomprising the intracellular region). The 189 or 191 amino acids(depending on the exact location of the signal cleavage site) ofACT-4-h-1 amino-proximal to the transmembrane segment are designated theextracellular domain, while the 37 amino acids carboxy-proximal to thetransmembrane segment are designated the intracellular domain. From theamino-terminus, the extracellular domain has an NH₂-terminal hydrophobicputative signal sequence, and three intrachain loops formed by disulfidebonding between paired cysteine residues.

The topological arrangement of ACT-4 receptor polypeptides is similar tothat of other members of the nerve growth factor receptor family,particularly to the rat OX-40 receptor. However, the other members showsome divergence in the number of extracellular disulfide loops andglycosylation sites and in the size of the intracellular domain. SeeMallet & Barclay, supra.

Although not all of the domains discussed above are necessarily presentin all ACT-4 receptor polypeptides, an extracellular domain is expectedto be present in most. Indeed, in some ACT-4 receptor polypeptides, itis possible that only an extracellular domain is present, and thenatural state of such proteins is not as cell-surface bound proteins,but as soluble proteins, for example, dispersed in an extracellular bodyfluid. The existence of soluble variant forms has been observed forother cell surface receptors, including one member of the nerve growthfactor receptor family, SFV-T2. See Mallet & Barclay, supra.

Besides substantially full-length polypeptides, the present inventionprovides for biologically active fragments of the polypeptides.Significant biological activities include receptor binding, antibodybinding (e.g., the fragment competes with an intact ACT-4 receptor forspecific binding to an antibody), immunogenicity (i.e., possession ofepitopes that stimulate B or T cell responses against the fragment), andagonism or antagonism of the binding of an ACT-4 receptor polypeptide toits ligand. A segment of an ACT-4 receptor protein or a domain thereofwill ordinarily comprise at least about 5, 7, 9, 11, 13, 16, 20, 40, or100 contiguous amino acids.

Segments of ACT-4 receptor polypeptides are often terminated nearboundaries of functional or structural domains. Structural andfunctional domains are identified by comparison of nucleotide and/oramino acid sequence data such as is shown in FIG. 5 to public orproprietary sequence databases. Preferably, computerized comparisonmethods are used to identify sequence motifs or predicted proteinconformation domains that occur in other proteins of known structureand/or function. Structural domains include an intracellular domain,transmembrane domain, and extracellular domain, which is in turncontains three disulfide-bonded loops. Functional domains include anextracellular binding domain through which the ACT-4 receptorpolypeptide interacts with external soluble molecules or othercell-bound ligands and an intracellular signal-transducing domain.

Some fragments will contain only extracellular domains, such as one ormore disulfide-bonded loops. Such fragments will often retain thebinding specificity of an intact ACT-4 receptor polypeptide, but will besoluble rather than membrane bound. Such fragments are useful ascompetitive inhibitors of ACT-4 receptor binding.

ACT-4 receptors are further identified by their status as members of thenerve growth factor receptor family. The amino acid sequence ofACT-4-h-l is at least 20% identical to NGF-R, TNF-R, CD40, 4-1BB, andfas/AP01. ACT-4-h-1 exhibits 62% amino acid sequence identity with therat OX-40 gene, which is also characterized by selective expression onactivated CD4⁺ cells.

ACT-4 receptors are also identified by a characteristic cellulardistribution. Most notably, ACT-4 receptors are usually easily detectedon activated CD4⁺ T cells (percent cells expressing usually greater thanabout 25 or 50% and often about 80%; mean channel fluorescence usuallygreater than about 10 and often about 20-25, on a Coulter Profile FlowCytometer after immunofluorescence staining). ACT-4 receptors areusually substantially absent on resting T-cells, B-cells (unlessactivated with PMA), NK cells, and monocytes (unless activated withPMA). Substantially absent means that the percentage of cells expressingACT-4 is usually less than about 5%, and more usually less than about2%, and that the mean channel is usually less than about 4, and moreusually less than about 2, measured on a Coulter Profile Flow Cytometer,after immunofluoresence staining of the cells. (See Example 2) ACT-4receptors are usually expressed at low levels on activated CD8⁺ cells(percent cells expressing about 4-10%; mean channel fluorescence about2-4 on a Coulter Profile Flow Cytometer after immunofluoresencestaining). The low level of expression observed on CD8⁺ cells suggeststhat expression is confined to a subpopulation of CD8⁺ cells. Theexpression of ACT-4 receptors on the surface of activated CD4⁺ cells hasbeen observed for several different mechanisms of activation, includingalloantigenic, tetanus toxoid or mitogenic (e.g., PHA) stimuli.Expression peaks after about 7 days of allogantigenic or tetanus toxoidstimulation and after about three days of PHA stimulation. These dataindicate that ACT-4 receptors should be classified as early activationantigens that are substantially absent on resting cells. The observationthat ACT-4 receptors are preferentially expressed on activated CD4⁺cells and are expressed to a much lesser extent on activated CD8⁺ cells,but are substantially absent on most or all other subtypes of lymphoidcells (except in response to highly nonphysiological stimuli such asPMA) contrasts with the cell type specificity of other activationantigens found on human leukocytes.

The expression of ACT-4 receptors on the surface of activated CD4⁺ Tcells suggests that the receptor has a role in activation of thesecells. Such a role is consistent with that of some other members of thenerve growth factor receptor family. For example, CD40 stimulates theG1-S phase transition in B lymphocytes, and nerve growth factor receptortransduces a signal from the cytokine nerve growth factor., whichresults in neuronal differentiation and survival (Barde, Y-A. Neuron 2:1525-1534 (1989)) (incorporated by reference for all purposes). However,other roles for ACT-4 receptors can also be envisaged, for example,interaction with other lymphoid cell types. The existence of such rolesis consistent with the diverse functions of other nerve growth factorreceptor family members, such as tumor necrosis factor, whoseinteraction with tumor necrosis factor receptor can result ininflammation or tumor cell death.

Fragments or analogs comprising substantially one or more functionaldomain (e.g., an extracellular domain) of ACT-4 receptors can be fusedto heterologous polypeptide sequences, such that the resultant fusionprotein exhibits the functional property(ies) conferred by the ACT-4receptor fragment and/or the fusion partner. The orientation of theACT-4 receptor fragment relative to the fusion partner will depend onexperimental considerations such as ease of construction, stability toproteolysis, thermal stability, immunological reactivity, amino- orcarboxyl-terminal residue modification, and so forth. Potential fusionpartners include chromogenic enzymes such as β-galactosidase, protein Aor G, a FLAG protein such as described by Blanar & Rutter, Science256:1014-1018 (1992), toxins (e.g., diphtheria toxin, Psuedonomasectotoxin A, ricin toxin or phospholipase C) and immunoglobulincomponents.

Recombinant globulins (Rg) formed by fusion of ACT-4 receptor fragmentsand immunoglobulin components often have most or all of thephysiological properties associated with the constant region of theparticular immunoglobulin class used. For example, the recombinantglobulins may be capable of fixing complement, mediating antibodydependent cell toxicity, stimulating B cells, or traversing blood vesselwalls and entering the interstitial space. The recombinant globulins areusually formed by fusing the C-terminus of an ACT-4 receptorextracellular domain to the N-terminus of the constant region domain ofa heavy chain immunoglobulin, thereby simulating the conformation of anauthentic immunoglobulin chain. The immunoglobulin chain is preferablyof human origin, particularly if the recombinant globulin is intendedfor therapeutic use. Recombinant globulins are usually soluble and havea number of advantageous properties relative to unmodified ACT-4receptors. These properties include prolonged serum half-life, thecapacity to lyse target cells for which an ACT-4 receptor has affinity,by effector functions, and the capacity to bind molecules such asprotein A and G, which can be used to immobilize the recombinantglobulin in binding analyses.

II. Methods of Producing Polypeptides

A. Recombinant Technologies

The nucleotide and amino acid sequences of ACT-4-h-1 shown in FIG. 5,and corresponding sequences for other ACT-4 receptor variants obtainedas described in Section III, infra, allow production of polypeptides offull-length ACT-4 receptor polypeptides sequences and fragments thereof.Such polypeptides may be produced in prokaryotic or eukaryotic hostcells by expression of polynucleotides encoding ACT-4 receptor, orfragments and analogs thereof. The cloned DNA sequences are expressed inhosts after the sequences have been operably linked to (i.e., positionedto ensure the functioning of) an expression control sequence in anexpression vector. Expression vectors are typically replicable in thehost organisms either as episomes or as an integral part of the hostchromosomal DNA. Commonly, expression vectors will contain selectionmarkers, e.g., tetracycline resistance or hygromycin resistance, topermit detection and/or selection of those cells transformed with thedesired DNA sequences (see, e.g., U.S. Pat. No. 4,704,362).

E. coli is one prokaryotic host useful for cloning the DNA sequences ofthe present invention. Other microbial hosts suitable for use includebacilli, such as Bacillus subtilis, and other Enterobacteriaceae, suchas Salmonella, Serratia, and various Pseudomonas species. In theseprokaryotic hosts, one can also make expression vectors, which willtypically contain expression control sequences compatible with the hostcell (e.g., an origin of replication). In addition, any number of avariety of well-known promoters will be present, such as the lactosepromoter system, a tryptophan (trp) promoter system, a beta-lactamasepromoter system, or a promoter system from phage lambda. The promoterswill typically control expression, optionally with an operator sequence,and have ribosome binding site sequences and the like, for initiatingand completing transcription and translation.

Other microbes, such as yeast, may also be used for expression.Saccharomyces is a preferred host, with suitable vectors havingexpression control sequences, such as promoters, including3-phosphoglycerate kinase or other glycolytic enzymes, and an origin ofreplication, termination sequences and the like as desired. Insect cells(e.g., SF9) with appropriate vectors, usually derived from baculovirus,are also suitable for expressing ACT-4 receptor or ligand polypeptides.See Luckow, et al. Bio/Technology 6:47-55 (1988) (incorporated byreference for all purposes).

Higher eukaryotic mammalian tissue cell culture may also be used toexpress and produce the polypeptides of the present invention (seeWinnacker, From Genes to Clones (VCH Publishers, NY, N.Y., 1987))(incorporated by reference for all purposes). Eukaryotic cells areactually preferred, because a number of suitable host cell lines capableof secreting and authentically modifying human proteins have beendeveloped in the art, and include the CHO cell lines, various COS celllines, HeLa cells, myeloma cell lines, Jurkat cells, etc. Expressionvectors for these cells can include expression control sequences, suchas an origin of replication, a promoter (e.g., a HSV tk promoter or pgk(phosphoglycerate kinase) promoter), an enhancer (Queen et al., Immunol.Rev. 89:49 (1986)), and necessary processing information sites, such asribosome binding sites, RNA splice sites, polyadenylation sites (e.g.,an SV40 large T Ag poly A addition site), and transcriptional terminatorsequences. Preferred expression control sequences are promoters derivedfrom immunoglobulin genes, SV40, adenovirus, bovine papillomavirus, andthe like. The vectors containing the DNA segments of interest (e.g.,polypeptides encoding an ACT-4 receptor) can be transferred into thehost cell by well-known methods, which vary depending on the type ofcellular host. For example, CaCl₂ transfection is commonly utilized forprokaryotic cells, whereas CaPO₄ treatment or electroporation may beused for other cellular hosts. Vectors may exist as episome orintegrated into the host chromosome.

B. Naturally Occurring ACT-4 Receptor Proteins

Natural ACT-4 receptor polypeptides are isolated by conventionaltechniques such as affinity chromatography. For example, polyclonal ormonoclonal antibodies are raised against previously-purified ACT-4-h-1and attached to a suitable affinity column by well known techniques.See, e.g., Hudson & Hay, Practical Immunology (Blackwell ScientificPublications, Oxford, UK, 1980), Chapter 8 (incorporated by referencefor all purposes). For example, anti-ACT-4-h-1 can be immobilized to aprotein-A sepharose column via crosslinking of the F_(c) domain with ahomobifunctional crosslinking agent, such as dimethyl pimelimidate. Cellextracts are then passed through the column, and ACT-4 receptor proteinspecifically bound by the column, eluted with, for example, 0.5 Mpyrogenic acid, pH 2.5. Usually, an intact form of ACT-4 receptor isobtained by such isolation techniques. Peptide fragments are generatedfrom intact ACT-4 receptors by chemical (e.g., cyanogen bromide) orenzymatic cleavage (e.g., V8 protease or trypsin) of the intactmolecule.

C. Other Methods

Alternatively, ACT-4 receptor polypeptides can be synthesized bychemical methods or produced by in vitro translation systems using apolynucleotide template to direct translation. Methods for chemicalsynthesis of polypeptides and in vitro translation are well known in theart, and are described further by Berger & Kimmel, Methods inEnzymology, Volume 152, Guide to Molecular Cloning Techniques AcademicPress, Inc., San Diego, Calif., 1987).

III. Nucleic Acids

A. Cloning ACT-4 Receptor Nucleic Acids

Example 5 presents nucleic acid sequence data for a cDNA clone of anACT-4 receptor designated ACT-4-h-1. The sequence includes both atranslated region and 3′ and 5′ flanking regions. This sequence data canbe used to design probes with which to isolate other ACT-4 receptorgenes. These genes include the human genomic gene encoding ACT-4-h-1,and cDNAs and genomic clones from higher mammalian species, and allelicand nonallelic variants, and natural and induced mutants of all of thesegenes. Specifically, all nucleic acid fragments encoding all ACT-4receptor polypeptides disclosed in this application are provided.Genomic libraries of many species are commercially available (e.g.,Clontech, Palo Alto, Calif.), or can be isolated de novo by conventionalprocedures. cDNA libraries are best prepared from activated CD4⁺ cells,which express ACT-4-h-1 in large amounts.

The probes used for isolating clones typically comprise a sequence ofabout at least 24 contiguous nucleotides (or their complement) of thecDNA sequence shown in FIG. 5. For example, a full-length polynucleotidecorresponding to the sequence shown in FIG. 5 can be labeled and used asa hybridization probe to isolate genomic clones from a human genomicclone library in e.g., λEMBL4 or λGEM11 (Promega Corporation, Madison,Wis.); typical hybridization conditions for screening plaque lifts(Benton & Davis, Science 196:180 (1978)) can be: 50% formamide, 5×SSC orSSPE, 1-5× Denhardt's solution, 0.1-1% SDS, 100-200 μg shearedheterologous DNA or tRNA, 0-10 % dextran sulfate, 1×10⁵ to 1×10⁷ cpm/mlof denatured probe with a specific activity of about 1×10⁸ cpm/μg, andincubation at 42° C. for about 6-36 hours. Prehybridization conditionsare essentially identical except that probe is not included andincubation time is typically reduced. Washing conditions are typically1-3×SSC, 0.1-1% SDS, 50-70° C. with change of wash solution at about5-30 minutes. Hybridization and washing conditions are typically lessstringent for isolation of higher cognate or nonallelic variants thanfor e.g., the human genomic clone of ACT-4-h-1.

Alternatively, probes can be used to clone ACT-4 receptor genes bymethods employing the polymerase chain reaction (PCR). Methods for PCRamplification are described in e.g., PCR Technology: Principles andApplications for DNA Amplification (ed. H. A. Erlich, Freeman Press, NY,N.Y., 1992); PCR Protocols: A Guide to Methods and Applications (eds.Innis, et al., Academic Press, San Diego, Calif., 1990); Mattila et al.,Nucleic Acids Res. 19:4967 (1991); Eckert, K. A. and Kunkel, T. A., PCRMethods and Applications 1:17 (1991); PCR (eds. McPherson et al., IRLPress, Oxford); and U.S. Pat. No. 4,683,202 (each of which isincorporated by reference for all purposes).

Alternatively, synthetic polynucleotide sequences corresponding to allor part of the sequences shown in FIG. 5 may be constructed by chemicalsynthesis of oligonucleotides.

Nucleotide substitutions, deletions, and additions can be incorporatedinto the polynucleotides of the invention. Nucleotide sequence variationmay result from degeneracy of the genetic code, from sequencepolymorphisms of various ACT-4 receptor alleles, minor sequencingerrors, or may be introduced by random mutagenesis of the encodingnucleic acids using irradiation or exposure to EMS, or by changesengineered by site-specific mutagenesis or other techniques of modernmolecular biology. See Sambrook et al., Molecular Cloning: A LaboratoryManual (C.S.H.P. Press, NY 2d ed., 1989) (incorporated by reference forall purposes). For nucleotide sequence that are capable of beingtranscribed and translated to produce a functional polypeptide,degeneracy of the genetic code results in a number of nucleotidesequences that encode the same polypeptide. The invention includes allsuch sequences. Generally, nucleotide substitutions, deletions, andadditions should not substantially disrupt the ability of an ACT-4receptor polynucleotide to hybridize to the sequence of ACT-4-h-1 shownin FIG. 5 under stringent conditions. Typically, ACT-4 receptorpolynucleotides comprise at least 25 consecutive nucleotides which aresubstantially identical to a naturally-occurring ACT-4 receptor sequence(e.g., FIG. 5), more usually ACT-4 receptor polynucleotides comprise atleast 50 to 100 consecutive nucleotides, which are substantiallyidentical to a naturally-occurring ACT-4 receptor sequence.

ACT-4 receptor polynucleotides can be short oligonucleotides (e.g.,about 10, 15, 25, 50 or 100 contiguous bases from the ACT-h-1 sequenceshown in FIG. 5), such as for use as hybridization probes and PCR (orLCR) primers. ACT-4 receptor polynucleotide sequences can also comprisepart of a larger polynucleotide that includes sequences that facilitatetranscription (expression sequences) and translation of the codingsequences, such that the encoded polypeptide product is produced.Construction of such polynucleotides is well known in the art and isdescribed further in Sambrook et al., supra (C.S.H.P. Press, NY 2d ed.1989). The ACT-4 receptor polynucleotide can be fused in frame withanother polynucleotide sequence encoding a different protein (e.g.,glutathione S-transferase, β-galactosidase or an immunoglobulin F_(C)domain) for encoding expression of a fusion protein (see, e.g., Byrn etal., Nature, 344:667-670 (1990)) (incorporated by reference for allpurposes).

IV. Antibodies and Hybridomas

In another embodiment of the invention, antibodies against ACT-4receptors and to their ligands (see Section V) are provided.

A. General Characteristics of Antibodies

Antibodies or immunoglobulins are typically composed of four covalentlybound peptide chains. For example, an IgG antibody has two light chainsand two heavy chains. Each light chain is covalently bound to a heavychain. In turn each heavy chain is covalently linked to the other toform a “Y” configuration, also known as an immunoglobulin conformation.Fragments of these molecules, or even heavy or light chains alone, maybind antigen. Antibodies, fragments of antibodies, and individual chainsare also referred to herein as immunoglobulins.

A normal antibody heavy or light chain has an N-terminal (NH₂) variable(V) region, and a C-terminal (—COOH) constant (C) region. The heavychain variable region is referred to as V_(H) (including, for example,V_(γ)), and the light chain variable region is referred to as V_(L)(including V_(κ) or V_(γ)). The variable region is the part of themolecule that binds to the antibody's cognate antigen, while the Fcregion (the second and third domains of the C region) determines theantibody's effector function (e.g., complement fixation, opsonization).Full-length immunoglobulin or antibody “light chains” (generally about25 kDa, about 214 amino acids) are encoded by a variable region gene atthe N-terminus (generally about 110 amino acids) and a κ (kappa) or λ(lambda) constant region gene at the COOH-terminus. Full-lengthimmunoglobulin or antibody “heavy chains” (generally about 50 Kd, about446 amino acids), are similarly encoded by a variable region gene(generally encoding about 116 amino acids) and one of the constantregion genes, e.g., gamma (encoding about 330 amino acids). Typically,the “V_(L)” will include the portion of the light chain encoded by theV_(L) and/or J_(L) (J or joining region) gene segments, and the “V_(H)”will include the portion of the heavy chain encoded by the V_(H), and/orD_(H) (D or diversity region) and J_(H) gene segments. See, generally,Roitt et al., Immunology (2d ed. 1989), Chapter 6 and Paul, FundamentalImmunology (Raven Press, 2d ed., 1989) (each of which is incorporated byreference for all purposes).

An immunoglobulin light or heavy chain variable region consists of a“framework” region interrupted by three hypervariable regions, alsocalled complementarity-determining regions or CDRs. The extent of theframework region and CDRs have been defined (see Kabat et al. (1987),“Sequences of Proteins of Immunological Interest,” U.S. Department ofHealth and Human Services; Chothia et al., J. Mol. Biol. 196:901-917(1987) (each of which is incorporated by reference for all purposes).The sequences of the framework regions of different light or heavychains are relatively conserved within a species. The framework regionof an antibody, that is the combined framework regions of theconstituent light and heavy chains, serves to position and align theCDRs in three dimensional space. The CDRs are primarily responsible forbinding to an epitope of an antigen. The CDRs are typically referred toas CDR1, CDR2, and CDR3, numbered sequentially starting from theN-terminus.

The constant region of the heavy chain molecule, also known as C_(H),determines the isotype of the antibody. Antibodies are referred to asIgM, IgD, IgG, IgA, and IgE depending on the heavy chain isotype. Theisotypes are encoded in the mu (μ), delta (Δ), gamma (γ), alpha (α), andepsilon (ε) segments of the heavy chain constant region, respectively.In addition, there are a number of ε subtypes. There are two types oflight chains, κ and λ. The determinants of these subtypes typicallyreside in the constant region of the light chain, also referred to asthe C_(L) in general, and C_(κ) or C_(λ) in particular.

The heavy chain isotypes determine different effector functions of theantibody, such as opsonization or complement fixation. In addition, theheavy chain isotype determines the secreted form of the antibody.Secreted IgG, IgD, and IgE isotypes are typically found in single unitor monomeric form. Secreted IgM isotype is found in pentameric form;secreted IgA can be found in both monomeric and dimeric form.

B. Production of Antibodies

Antibodies which bind either an ACT-4 receptor, a ligand thereto, orbinding fragments of either, can be produced by a variety of means. Theproduction of non-human monoclonal antibodies, e.g., murine, rat and soforth, is well known and may be accomplished by, for example, immunizingthe animal with a preparation containing an ACT-4 receptor or itsligands, or an immunogenic fragment of either of these. Particularly,useful as immunogens are cells stably transfected with recombinant ACT-4receptor genes and expressing ACT-4 receptors on their cell surface.Antibody-producing cells obtained from the immunized animals areimmortalized and screened for the production of an antibody which bindsto ACT-4 receptors or their ligands. See Harlow & Lane, Antibodies, ALaboratory Manual (C.S.H.P. NY, 1988) (incorporated by reference for allpurposes).

Several techniques for generation of human monoclonal antibodies havealso been described but are generally more onerous than murinetechniques and not applicable to all antigens. See, e.g., Larrick etal., U.S. Pat. No. 5,001,065, for review (incorporated by reference forall purposes). One technique that has successfully been used to generatehuman monoclonal antibodies against a variety of antigens is the triomamethodology of Ostberg et al. (1983), Hybridoma 2:361-367, Ostberg, U.S.Pat. No. 4,634,664, and Engleman et al., U.S. Pat. No. 4,634,666(incorporated by reference for all purposes). The antibody-producingcell lines obtained by this method are called triomas, because they aredescended from three cells—two human and one mouse. Triomas have beenfound to produce antibody more stably than ordinary hybridomas made fromhuman cells.

An alternative approach is the generation of humanized immunoglobulinsby linking the CDR regions of non-human antibodies to human constantregions by recombinant DNA techniques. See Queen et al., Proc. Natl.Acad. Sci. USA 86:10029-10033 (1989) and WO 90/07861 (incorporated byreference for all purposes). The humanized immunoglobulins have variableregion framework residues substantially from a human immunoglobulin(termed an acceptor immunoglobulin) and complementarity determiningregions substantially from a mouse immunoglobulin, e.g., the L106antibody (referred to as the donor immunoglobulin). The constantregion(s), if present, are also substantially from a humanimmunoglobulin. The human variable domains are usually chosen from humanantibodies whose framework sequences exhibit a high degree of sequenceidentity with the murine variable region domains from which the CDRswere derived. The heavy and light chain variable region frameworkresidues can be derived from the same or different human antibodysequences. The human antibody sequences can be the sequences ofnaturally occurring human antibodies or can be consensus sequences ofseveral human antibodies. See Carter et al., WO 92/22653. Certain aminoacids from the human variable region framework residues are selected forsubstitution based on their possible influence on CDR conformationand/or binding to antigen. Investigation of such possible influences isby modeling, examination of the characteristics of the amino acids atparticular locations, or empirical observation of the effects ofsubstitution or mutagenesis of particular amino acids.

For example, when an amino acid differs between a murine L106 variableregion framework residue and a selected human variable region frameworkresidue, the human framework amino acid should usually be substituted bythe equivalent framework amino acid from the mouse antibody when it isreasonably expected that the amino acid:

(1) noncovalently binds antigen directly,

(2) is adjacent to a CDR region,

(3) otherwise interacts with a CDR region (e.g. is within about 3A of aCDR region), or

(3) participates in the VL-VH interface.

Other candidates for substitution are acceptor human framework aminoacids that are unusual for a human immunoglobulin at that position.These amino acids can be substituted with amino acids from theequivalent position of the L106 antibody or from the equivalentpositions of more typical human immunoglobulins.

A further approach for isolating DNA sequences which encode a humanmonoclonal antibody or a binding fragment thereof is by screening a DNAlibrary from human B cells according to the general protocol outlined byHuse et al., Science 246:1275-1281 (1989) and then cloning andamplifying the sequences which encode the antibody (or binding fragment)of the desired specificity. The protocol described by Huse is renderedmore efficient in combination with phage display technology. See, e.g.,Dower et al., WO 91/17271 and McCafferty et al., WO 92/01047. Phagedisplay technology can also be used to mutagenize CDR regions ofantibodies previously shown to have affinity for ACT-4 receptors ortheir ligands. Antibodies having improved binding affinity are selected.

Anti-ACT-4 receptor antibodies that specifically bind to the sameepitope as the L106 antibody are usually identified by a competitivebinding assay. The assay has three components, an ACT-4 polypeptide(e.g., ACT-4-h-1), L106 antibody, which is usually labelled, and theantibody under test. Often the ACT-4 receptor polypeptide is immobilizedto a solid support. The test antibody binds to the same epitope as theL106 antibody if it reduces the amount of L106 antibody thatspecifically binds to the ACT-4 receptor polypeptide. The extent ofscreening necessary to obtain such antibodies can be reduced bygenerating antibodies with a protocol in which the specific epitopebound by L106 is used as an immunogen. Antibodies binding to the sameepitope as L106 may exhibit a substantially, but not completely,identical amino acid sequence to the L106 antibody, or may have anunrelated primary structure to the L106 antibody.

Anti-ACT-4 receptor antibodies having a different binding specificitythan L106 (i.e., which bind to a different epitope) are identified by acomplementary approach. Test antibodies are screened for failure tocompete with the L106 antibody for binding to an ACT-4 receptorpolypeptide. The extent of screening can be reduced by generatingantibodies with a protocol in which a fragment lacking a specificepitope bound by L106 is used as an immunogen.

Antibodies having the capacity to stimulate or inhibit activation ofCD4⁺ cells can be identified by the screening procedures discussed inSection VI, infra. Some antibodies may selectively inhibit activation inresponse to some stimuli (e.g., alloantigenic but not mitogenic, or viceversa), and not to others. Some antibodies' inhibitory capacity maydepend on the time after activation at which the antibody is added. Someantibodies may have the capacity to activate CD4⁺ cells independently ofother stimuli, whereas other anti-ACT-4 receptor antibodies may onlyhave the capacity to augment the efficacy of another stimulus such asthat provided by PHA.

Antibodies isolated by the above procedures can be used to generateanti-idiotypic antibodies by, for example, immunization of an animalwith the primary antibody. For anti-ACT-4 receptor antibodies,anti-idiotype antibodies whose binding to the primary antibody isinhibited by ACT-4 receptors or fragments thereof are selected. Becauseboth the anti-idiotypic antibody and the ACT-4 receptors or fragmentsthereof bind the primary immunoglobulin, the anti-idiotypicimmunoglobulin may represent the “internal image” of an epitope and thusmay substitute for the ACT-4 ligand.

C. Epitope Mapping

The epitope bound by the L106 or any other anti-ACT-4 receptor antibodyis determined by providing a family of fragments containing differentamino acid segments from an ACT-4 receptor polypeptide, such asACT-4-h-1. Each fragment typically comprises at least 4, 6, 8, 10, 20,50 or 100 contiguous amino acids. Collectively, the family ofpolypeptide covers much or all of the amino acid sequence of afull-length ACT-4 receptor polypeptide. Members of the family are testedindividually for binding to e.g., the L106 antibody. The smallestfragment that can specifically bind to the antibody under testdelineates the amino acid sequence of the epitope recognized by theantibody.

D. Fragments of Antibodies, and Immunotoxins

In another embodiment of the invention, fragments of antibodies againstACT-4 receptors or their ligands are provided. Typically, thesefragments exhibit specific binding to the ACT-4 receptor with anaffinity of at least 10⁷ M, and more typically 10⁸ or 10⁹ M. Antibodyfragments include separate heavy chains, light chains Fab, Fab′ F(ab′)₂,Fabc, and Fv. Fragments are produced by recombinant DNA techniques, orby enzymic or chemical separation of intact immunoglobulins.

In another embodiment, immunotoxins are provided. An immunotoxin is achimeric compound consisting of a toxin linked to an antibody having adesired specificity. The antibody serves as a targeting agent for thetoxin. See generally Pastan et al., Cell 47:641-648 (1986). A toxinmoiety is couple to an intact antibody or a fragment thereof by chemicalor recombinant DNA techniques. Preferably, the toxin is linked to animmunoglobulin chain in the form of a contiguous protein. See, e.g.Chovnick et al., Cancer Res. 51:465; Chaudhary et al., Nature 339:394(1989) (incorporated by reference for all purposes). Examples ofsuitable toxin components are listed in Section I, supra, and arereviewed in e.g., The Specificity and Action of Animal, Bacterial andPlant Toxins (ed. P. Cuatrecasas, Chapman Hall, London, 1976)(incorporated by reference for all purposes).

E. Hybridomas and Other Cell Lines

All hybridomas, triomas and other cell lines producing the antibodiesand their fragments discussed, supra, are expressly included in theinvention. These include the hybridoma line HBL106, deposited asATCC______ , which produces the L106 mouse antibody.

F. Uses of Antibodies

Anti-ACT-4 receptor antibodies and their binding fragments are usefulfor screening cDNA expression libraries, preferably containing human orprimate cDNA derived from various tissues and for identifying clonescontaining cDNA inserts, which encode structurally-related,immunocrossreactive proteins. See Aruffo & Seed, Proc. Natl. Acad. Sci.USA 84:8573-8577 (1987) (incorporated by reference for all purposes).Antibodies are also useful to identify and/or purify immunocrossreactiveproteins that are structurally or evolutionarily related to the nativeACT-4 receptor polypeptides or to fragments thereof used to generate theantibody. Antibodies against ACT-4 ligands are analogously useful inisolating further ligands and variants thereof. Diagnostic andtherapeutic uses of antibodies, binding fragments thereof, immunotoxinsand idiotypic antibodies are described in Section VII, infra.

V. ACT-4 Ligands

The term ACT-4 ligand is used to denote a protein that specificallybinds to an ACT-4 receptor polypeptide and that is capable of forming acomplex with such a polypeptide, at least in part, by noncovalentbinding. Ligands can be naturally-occurring or synthetic molecules, andcan be in soluble form or anchored to the surface of a cell. Multipledifferent ligands may bind the same ACT-4 receptor. Conversely, oneligand may bind to more than one ACT-4 receptor. The term “ACT-4 ligand”does not usually include antibodies to ACT-4 receptor polypeptides.Usually, binding of a ligand to an ACT-4 receptor will initiate a signalthat alters the physical and/or functional phenotype of a cell bearingthe ACT-4 receptor and/or a cell bearing the ACT-4 ligand. Antibodiesagainst either ACT-4 or its ligands can have the capacity to block orstimulate signal transduction. It will, of course, be recognized thedesignation of ACT-4 as a receptor and its specific binding partner as aligand is somewhat arbitrary and might, in some circumstances, bereversed.

ACT-4 ligands are expected to share some of the properties of otherligands which bind to members of the nerve growth factor receptorsuperfamily. These ligands include the cytokines TNF-β, TNF-β, CD40-L,CD-27-L and CD30-L. With the exception of TNF-β, these ligands existboth as type II integral membrane cell surface proteins and as solubleproteins. The extracellular domains of these ligands consist of about150 amino acids and form several β-pleated sheets, which assemble into aslitted cylindrical structure (termed a “jelly role” by Bazan et al.,Current Biology 3:603-606 (1993)) (incorporated by reference for allpurposes).

Source materials for supplying ACT-4 ligands are identified by screeningdifferent cell types, particularly lymphoid and hematopoietic cells,bodily fluids and tissue extracts, with labelled ACT-4 receptor,preferably in soluble form, as a probe. Often, the ACT-4 receptor or abinding fragment thereof is fused to a second protein for purposes ofscreening. Particularly suitable are recombinant globulins formed byfusing the extracellular portion of ACT-4 to the constant region of animmunoglobulin heavy chain.

ACT-4 ligands are purified from cells or other biological materialsidentified by this screening method using techniques of classicalprotein chemistry. Such techniques include selective precipitation withsuch substances as ammonium sulfate, column chromatography,immunopurification methods, and others. See, e.g., R. Scopes, ProteinPurification: Principles and Practice (Springer-verlag, NY, 1982)(incorporated by reference for all purposes). Usually, purificationprocedures will include an affinity chromatography step in which anACT-4 polypeptide or a binding fragment thereof is used as theimmobilized reagent. ACT-4-constant regions can be convenientlyimmobilized by binding of the constant region moiety to protein A or G.ACT-4 ligands can also be purified using anti-idiotypic antibodies toACT-4 receptors as the affinity reagent.

To determine the amino acid sequence or to obtain polypeptide fragmentsof the receptor, the receptor may be digested with trypsin. Peptidefragments may be separated by reversed-phase high performance liquidchromatography (HPLC) and analyzed by gas-phase sequencing. Othersequencing methods known in the art may also be used. The sequence datacan be used to design degenerate probes for isolation of cDNA or genomicclones encoding ACT-4 ligands.

Alternatively, cDNA clones encoding ACT-4 ligands can be obtained byexpression cloning. In this approach, a cDNA library is prepared fromcells expressing an ACT-4 ligand (identified as discussed, supra). Thelibrary is expressed in appropriate cells (e.g., COS-7), and clonesbearing the ACT-4 ligand are identified by screening with labelled ACT-4or binding fragment thereof, optionally fused to a constant domain of animmunoglobulin heavy chain.

The ACT-4 ligands or their binding domains can be used to affinitypurify respective ACT-4 receptors. ACT-4 ligands and binding fragmentsthereof are also useful as agonists or antagonists of ACT-4 ligandbinding, and can be used in the therapeutic methods discussed in SectionVII, infra. For membrane-bound ACT-4 ligands, binding fragments willcomprise part of the extracellular domain of an ACT-4 receptor. ACT-4ligands and fragments thereof are also useful in screening assays foridentifying agonists and antagonists of ACT-4 and/or its ligand. ACT-4ligands can be fused to other protein, such as toxins and immunoglobulinconstant domains, as discussed, supra, for ACT-4 receptors.

VI. Screening for Agonists and Antagonists ACT-4 receptor and ACT-4ligand fragments, analogs thereof, antibodies and anti-idiotypicantibodies thereto, as well as other chemical or biological agents arescreened for their ability to block or enhance binding of an ACT-4ligand to its receptor. In addition, they are tested for their abilityto stimulate or inhibit metabolic processes, such as DNA synthesis orprotein phosphorylation in cells bearing either an ACT-4 receptor or anACT-4 ligand anchored to their surfaces.

In some methods, the compound under test is screened for its ability toblock or enhance binding of a purified binding fragment of an ACT-4receptor (or fusion protein thereof) to a purified binding fragment ofan ACT-4 ligand (or fusion protein thereof). In such experiments, eitherthe receptor or ligand fragment is usually immobilized to a solidsupport. The test compound then competes with an ACT-4 ligand orreceptor fragment (whichever is not attached to the support) for bindingto the support. Usually, either the test compound or the competingligand or receptor is labelled.

In other methods, either or both of the ACT-4 receptor and ligand, orbinding fragments of these molecules, are expressed on a cell surface.For example, ACT-4-h-1 antigen is expressed from recombinant DNA ine.g., COS-7 cells (see Example 6). In these methods, the existence ofagonism or antagonism is determined from the degree of binding betweenan ACT-4 receptor and its ligand that occurs in the presence of the testcompound. Alternatively, activity of the test compound is assayed bymeasurement of ³H-thymidine incorporation into DNA or ³²p incorporationinto proteins in cells bearing an ACT-4 receptor and/or cells bearing anACT-4 ligand.

Compounds that block ACT-4-induced DNA synthesis or proteinphosphorylation are antagonists. Compounds that activate DNA synthesisor phosphorylation via interaction with an ACT-4 receptor or its ligandare agonists. Agonistic or antagonistic activity can also be determinedfrom other functional or physical endpoints of leukocyte activation, orfrom clinically desirable or undesirable outcomes, such as cytolyticactivity, or extravasation of leukocytes into tissues from bloodvessels.

The ability of agents to agonize or antagonize T-cell proliferation invitro can be correlated with the ability to affect the immune responsein vivo. In vivo activity is typically assayed using suitable animalmodels such as mice or rats. To assay the effect of agents on allograftrejection, for example, potential therapeutic agents can be administeredto the animals at various times before introduction of the allogeneictissue; and the animals can be monitored for graft rejection. Suitablemethods for performing the transplant and monitoring for graft rejectionhave been described (see, e.g., Hislop et al., J. Thorac. Cardiovasc.100:360-370 (1990)) (incorporated by reference for all purposes).

VII. Therapeutic and Diagnostic Methods and Compositions

A. Diagnostic Methods

Diseases and conditions of the immune system associated with an alteredabundance, or functional mutation, of an ACT-4 receptor or its mRNA, oran ACT-4 ligand or its mRNA may be diagnosed using the probes and/orantibodies of the present invention. The provision of antibodies againstthe ACT-4 receptor and nucleic acid probes complementary to its mRNAallows activated CD4⁺ T-cells to be distinguished from other leukocytesubtypes. The presence of such cells is indicative of a MHC class IIinduced immune response against, e.g., invading bacteria. Comparison ofthe numbers of activated CD4⁺ cells and CD8⁺ cells may allowdifferential diagnosis between bacterial and viral infections, whichpredominantly induce these respective activated cell types. The presenceof activated CD4⁺ cells is also indicative of undesirable diseases andconditions of the immune system, such as allograft rejection, graftversus host disease, autoimmune diseases, allergies and inflammation.The efficacy of therapeutic agents in treating such diseases andconditions can be monitored.

Diagnosis can be accomplished by removing a cellular sample (e.g., bloodsample, lymph node biopsy or tissue) from a patient. The sample is thensubjected to analysis for determining: (1) the amount of expressed ACT-4receptor or ligand in individual cells of the sample (e.g., byimmunohistochemical staining of fixed cells with an antibody or FACS™analysis), (2) the amount of ACT-4 receptor or ligand mRNA in individualcells (by in situ hybridization with a labelled complementarypolynucleotide probe), (3) the amount of ACT-4 receptor or ligand mRNAin the cellular sample by RNA extraction followed by hybridization to alabeled complementary polynucleotide probe (e.g., by Northern blotting,dot blotting, solution hybridization or quantitative PCR), or (4) theamount of ACT-4 receptor or ligand in the cellular sample (e.g., by celldisruption followed by immunoassay or Western blotting of the resultantcell extract).

Diagnosis can also be achieved by in vivo administration of a diagnosticreagent (e.g., a labelled anti-ACT-4 receptor antibody for diagnosis ofactivated CD4⁺ T-cells) and detection by in vivo imaging. Theconcentration of diagnostic agent administered should be sufficient thatthe binding to those cells having the target antigen is detectablecompared to the background signal. Further, it is desirable that thediagnostic reagent can be rapidly cleared from the circulatory system inorder to give the best target-to-background signal ratio. The diagnosticreagent can be labelled with a radioisotope for camera imaging, or aparamagnetic isotope for magnetic resonance or electron spin resonanceimaging.

A change (typically an increase) in the level of protein or mRNA of anACT-4 receptor or ligand in a cellular sample from an individual, whichis outside the range of clinically established normal levels, mayindicate the presence of an undesirable immune reaction in theindividual from whom the sample was obtained, and/or indicate apredisposition of the individual for developing (or progressing through)such a reaction. Protein or mRNA levels may be employed as adifferentiation marker to identify and type cells of certain lineages(i.e., activated CD4⁺ cells for the ACT-4 receptor) and developmentalorigins. Such cell-type specific detection may be used forhistopathological diagnosis of undesired immune responses.

B. Diagnostic Kits

In another aspect of the invention, diagnostic kits are provided for thediagnostic methods described supra. The kits comprise container(s)enclosing the diagnostic reagents, such as labelled antibodies againstACT-4 receptors, and reagents and/or apparatus for detecting the label.Other components routinely found in such kits may also be includedtogether with instructions for performing the test.

C. Pharmaceutical Compositions

The pharmaceutical compositions used for prophylactic or therapeutictreatment comprise an active therapeutic agent, for example, an ACT-4receptor, ligand, fragments thereof, and antibodies and idiotypicantibodies thereto, and a variety of other components. The preferredform depends on the intended mode of administration and therapeuticapplication. The compositions may also include, depending on theformulation desired, pharmaceutically-acceptable, non-toxic carriers ordiluents, which are defined as vehicles commonly used to formulatepharmaceutical compositions for animal or human administration. Thediluent is selected so as not to affect the biological activity of thecombination. Examples of such diluents are distilled water,physiological saline, Ringer's solutions, dextrose solution, and Hank'ssolution. In addition, the pharmaceutical composition or formulation mayalso include other carriers, adjuvants, or nontoxic, nontherapeutic,nonimmunogenic stabilizers and the like.

D. Therapeutic Methods

The therapeutic methods employ the therapeutic agents discussed abovefor treatment of various diseases in humans or animals, particularlyvertebrate mammals. The therapeutic agents include ACT-4 receptors,binding fragments thereof, ACT-4 ligands, binding fragments thereof,anti-ACT-4 receptor and ligand antibodies and anti-idiotypic antibodiesthereto, binding fragments of these antibodies, humanized versions ofthese antibodies, immunotoxins, and other agents discussed, supra. Sometherapeutic agents function by blocking or otherwise antagonizing theaction of an ACT-4 receptor with its ligand. Other therapeutic agentsfunction by killing cells bearing a polypeptide against which the agentis targeted. For example, anti-ACT-4 receptor antibodies with effectorfunctions or which are conjugated to toxins, radioisotopes or drugs arecapable of selectively killing activated CD4⁺ T-cells. Selectiveelimination of such cells is particularly advantageous because anundesirable immune response can be reduced or eliminated, whilepreserving a residual immune capacity in the form of inactivated CD4⁺cells and CD8⁺ cells to combat invading microorganisms to which apatient may subsequently be exposed. Other therapeutic agents functionas agonists of the interaction between the ACT-4 receptor and ligand.

1. Dosages and Methods of Administration

In therapeutic applications, a pharmaceutical composition (e.g.,comprising an anti-ACT-4 receptor antibody) is administered, in vivo orex vivo, to a patient already suffering from an undesirable immuneresponse (e.g., transplant rejection), in an amount sufficient to cure,partially arrest, or detectably slow the progression of the condition,and its complications. An amount adequate to accomplish this is definedas a “therapeutically effective dose” or “efficacious dose.” Amountseffective for this use will depend upon the severity of the condition,the general state of the patient, and the route of administration, andcombination with other immunosuppressive drugs, if any, but generallyrange from about 10 ng to about 1 g of active agent per dose, withsingle dosage units of from 10 mg to 100 mg per patient being commonlyused. Pharmaceutical compositions can be administered systemically byintravenous infusion, or locally by injection. The latter isparticularly useful for localized undesired immune response such as hostversus graft rejection. For a brief review of methods for drug delivery,see Langer, Science 249:1527-1533 (1990) (incorporated by reference forall purposes).

In prophylactic applications, pharmaceutical compositions areadministered to a patients at risk of, but not already suffering anundesired immune reaction (e.g., a patient about to undergo transplantsurgery). The amount of antibody to be administered is a“prophylactically effective dose,” the precise amounts of which willdepend upon the patient's state of health and general level of immunity,but generally range from 10 ng to 1 g per dose, especially 10 mg to 100mg per patient.

Because the therapeutic agents of the invention are likely to be moreselective and generally less toxic than conventional immunomodulatingagents, they will be less likely to cause the side effects frequentlyobserved with the conventional agents. Moreover, because some of thetherapeutic agents are human protein sequences (e.g., binding fragmentsof an ACT-4 receptor or ligand or humanized antibodies), they are lesslikely to cause immunological responses such as those observed withmurine anti-CD3 antibodies. The therapeutic agents of the presentinvention can also be combined with traditional therapeutics, and can beused to lower the dose of such agents to levels below those associatedwith side effects. For example, other immunosuppressive agents such asantibodies to the α3 domain, T cell antigens (e.g., OKT4 and OKT3),antithymocyte globulin, as well as chemotherapeutic agents such ascyclosporine, glucocorticoids, azathioprine, prednisone can be used inconjunction with the therapeutic agents of the present invention.

For destruction of a specific population of target cells, it can beadvantageous to conjugate the therapeutic agents of the presentinvention to another molecule. For example, the agents can be joined toliposomes containing particular immunosuppressive agents, to a specificmonoclonal antibody or to a cytotoxin or other modulator of cellularactivity, whereby binding of the conjugate to a target cell populationwill result in alteration of that population. A number of protein toxinshave been discussed supra. Chemotherapeutic agents include, for example,doxorubicin, daunorubicin, methotrexate, cytotoxin, and anti-sense RNA.Antibiotics can also be used. In addition, radioisotopes such asyttrium-90, phosphorus-32, lead-212, iodine-131, or palladium-109 can beused. The emitted radiation destroys the targeted cells.

2. Diseases and Conditions Amenable to Treatment

The pharmaceutical compositions discussed above are suitable fortreating several diseases and conditions of the immune system.

a. Transplant Rejection

Over recent years there has been a considerable improvement in theefficiency of surgical techniques for transplanting tissues and organssuch as skin, kidney, liver, heart, lung, pancreas and bone marrow.Perhaps the principal outstanding problem is the lack of satisfactoryagents for inducing immunotolerance in the recipient to the transplantedallograft or organ. When allogeneic cells or organs are transplantedinto a host (i.e., the donor and donee are different individual from thesame species), the host immune system is likely to mount an immuneresponse to foreign antigens in the transplant (host-versus-graftdisease) leading to destruction of the transplanted tissue. CD8⁺ cells,CD4⁺ cells and monocytes are all involved in the rejection of transplanttissues. The therapeutic agents of the present invention are useful toblock alloantigen-induced immune responses in the donee (e.g., blockageor elimination of allogen-activation of CD4⁺ T-cells by anti-ACT-4receptor antibodies) thereby preventing such cells from participating inthe destruction of the transplanted tissue or organ.

b. Graft Versus Host Disease

A related use for the therapeutic agents of the present invention is inmodulating the immune response involved in “graft versus host” disease(GVHD). GVHD is a potentially fatal disease that occurs whenimmunologically competent cells are transferred to an allogeneicrecipient. In this situation, the donor's immunocompetent cells mayattack tissues in the recipient. Tissues of the skin, gut epithelia andliver are frequent targets and may be destroyed during the course ofGVHD. The disease presents an especially severe problem when immunetissue is being transplanted, such as in bone marrow transplantation;but less severe GVHD has also been reported in other cases as well,including heart and liver transplants. The therapeutic agents of thepresent invention are used to block activation of, or eliminate, thedonor T-cells (particularly activated CD4⁺ T-cells, for therapeuticagents targeted against the ACT-4 receptor), thereby inhibiting theirability to lyse target cells in the host.

c. Autoimmune diseases

A further situation in which immune suppression is desirable is intreatment of autoimmune diseases such as insulin-dependent diabetesmellitus, multiple sclerosis, stiff man syndrome, rheumatoid arthritis,myasthenia gravis and lupus erythematosus. In these disease, the bodydevelops a cellular and/or humoral immune response against one of itsown antigens leading to destruction of that antigen, and potentiallycrippling and/or fatal consequences. Activated CD4⁺ T-cells are believedto play a major role in many autoimmune diseases. Autoimmune diseasesare treated by administering one of the therapeutic agents of theinvention, particularly agents targeted against an ACT-4 receptor.Optionally, the autoantigen, or a fragment thereof, against which theautoimmune disease is targeted can be administered shortly before,concurrently with, or shortly after the immunosuppressive agent. In thismanner, tolerance can be induced to the autoantigen under cover of thesuppressive treatment, thereby obviating the need for continuedimmunosuppression. See, e.g., Cobbold et al., WO 90/15152 (1990).

d. Inflammation

Inflammation represents the consequence of capillary dilation withaccumulation of fluid and migration of phagocytic leukocytes, such asgranulocytes and monocytes. Inflammation is important in defending ahost against a variety of infections but can also have undesirableconsequences in inflammatory disorders, such as anaphylactic shock,arthritis and gout. Activated T-cells have an important modulatory rolein inflammation, releasing interferon γ and colony stimulating factorsthat in turn activate phagocytic leukocytes. The activated phagocyticleukocytes are induced to express a number of specific cells surfacemolecules termed homing receptors, which serve to attach the phagocytesto target endothelial cells. Inflammatory responses can be reduced oreliminated by treatment with the therapeutic agents of the presentinvention. For example, therapeutic agents targeted against the ACT-4receptor function by blocking activation of, or eliminating activated,CD4⁺ cells, thereby preventing these cells from releasing moleculesrequired for activation of phagocytic cell types.

e. Infectious Agents

The invention also provides methods of augmenting the efficacy ofvaccines in preventing or treating diseases and conditions resultingfrom infectious agents. Therapeutic agents having the capacity toactivate CD4⁺ T-cells (e.g., certain monoclonal antibodies against aACT-4-h-1 receptor polypeptide) are administered shortly before,concurrently with, or shortly after the vaccine containing a selectedantigen. The therapeutic agent serves to augment the immune responseagainst the selected antigen. These methods may be particularlyadvantageous in patients suffering from immune deficiency diseases.

The following examples are offered to illustrate, but not to limit, theinvention.

EXAMPLES Example 1

A Monoclonal Antibody Against ACT-4-h-1

Mice were immunized with PHA-transformed T-lymphoblasts. Splenocytesfrom immunized mice were fused with SP2/O myeloma cells and hybridomassecreting antibodies specific for the T-cell clone were selected. Thehybridomas were cloned by limiting dilution. A monoclonal antibody,designated L106, produced by one of the resulting hybridoma, wasselected for further characterization. The L106 antibody was found tohave an IgG1 isotype. A hybridoma producing the antibody, designatedHBL106 has been deposited at the American Type Culture Collectionat______, on______, and assigned ATCC Accession No.______.

Example 2

Cellular Distribution of Polypeptide Recognized by L106 Antibody

Samples containing the antibody L106 were made available to certainparticipants at the Fourth International Workshop and Conference onHuman Leucocyte Differentiation Antigens (Vienna 1989) for the purposeof identifying tissue and cell types which bind to the L106 antibody.The data from the workshop are presented in Leukocyte Typing IV (ed. W,Knapp, Oxford U. Press, 1989) (incorporated by reference for allpurposes) and an accompanying computer data base available from WalterR. Gilks, MRC Biostatistics Unit, Cambridge University, England. Thisreference reports the L106 antibody binds a polypeptide of about 50 kDa.This polypeptide was reported to be present on HUT-102 cells (atransformed T-cell line), PHA-activated peripheral blood lymphocytes, anEBV-transformed B-lymphoid cell line, and HTLV-II transformed T-cellline, PMA-activated tonsil cells, ConA- or PHA-activated PBLs, andPMA-activated monocytes. The polypeptide was reported to besubstantially absent on inter alia resting basophils, endothelial cells,fibroblasts, interferon γ-activated monocytes, peripheral non-T-cells,peripheral granulocytes, peripheral monocytes, peripheral mononuclearcells, peripheral T cells, and peripheral red blood cells.

The present inventors have obtained data indicating that the 50 kDapolypeptide (hereinafter “ACT-4-h-1 receptor”) is preferentiallyexpressed on the CD4⁺ subspecies of activated T-cells. In one series ofexperiments, cell-specific ACT-4-h-1 expression was analyzed onunfractionated PBLs by a two-color staining method. PBL were activatedwith PHA for about two days (using the culture conditions described inExample 3), and analyzed for cell-surface expression of ACT-4-h-1 ondifferent cellular subtypes by staining with two differently-labelledantibodies (FITC and PE labels). Labels were detected by FACS™ analysisessentially as described by Picker et al., J. Immunol. 150:1105-1121(1993) (incorporated by reference for all purposes). One antibody, L106,was specific for ACT-4-h-1, the other antibody was specific for aparticular leukocyte subtype. FIG. 1 shows three charts in which L106staining is shown on the Y-axis of each chart, and anti-CD4, anti-CD8and anti-CD19 staining as the X-axes of the respective charts. For thechart stained with anti-CD4, many cells appear as double positives(i.e., express both CD4 and ACT-4-h-1). For the chart stained withanti-CD8, far fewer cells appear as double positives. For the chartstained with anti-CD19 (a B-cell marker), double-positive cells aresubstantially absent.

In another series of experiments expression of ACT-4-h-1 was analyzed bysingle-color staining on isolated cell types. Cells were stained withfluorescently labelled L106 antibody and the label was detected by FACS™analysis. See Engleman et al., J. Immunol. 127:2124-2129 (1981)(incorporated by reference for all purposes). In some experiments, cellswere activated by PHA stimulation for about two days (again using theculture conditions described in Example 3). The results from thisexperiment, together with those from the two-color staining experimentdescribed supra, are summarized in Table 1. Table 1 shows that about 80%of activated CD4⁺ cells expressed ACT-4-h-1 with a mean channelfluorescence of >20, irrespective whether the CD4⁺ cells are isolated(one-color staining) or in unfractionated PBLs (two-color staining). Thelevel of expression of ACT-4-h-1 on activated CD8⁺ cells is much lowerthan on activated CD4⁺ T-cells in the two-color staining experiment, andvery much lower in the one-color staining. Thus, the extent ofexpression on activated CD8⁺ cells appears to depend on whether the C8⁺cells are fractionated from other PBLs before activation. Inunfractionated CD8⁺ cells (two-color staining), about 10% of cellsexpress ACT-4-h-1, with a mean channel fluorescence of about 4. In thefractionated cells, only about 4% of cells express ACT-4-h-1 with a meanchannel fluorescence of about 2. These data suggest that ACT-4-h-1 isexpressed only on a small subtype of activated CD8⁺ cells and that thissubtype is somewhat more prevalent when the CD8⁺ cells are activated inthe presence of other PBLs.

Table 1 also indicates that ACT-4-h-1 was substantially absent on allresting leukocyte subtypes tested (i.e., CD4⁺ T-cells, CD8⁺ T-cells,CD19⁺ B-cells, CD14⁺ monocytes, granulocytes and platelets), and wasalso substantially absent on activated B-cells and monocytes. ACT-4-h-1was also found to be substantially absent on most tumor cell linestested. However, Molt3, Raji and NC37 cell lines did show a low level ofexpression. TABLE 1 CELL SPECIFICITY OF ACT-4-h-1 EXPRESSION Expressionof ACT-4-h-1 % Cells MCF¹ Two Color Staining CD4⁺ T-Cells (resting) <2<2 CD4⁺ T-Cells (activated)² 80 25 CD8⁺ T-Cells (resting) <2 <2 CD8⁺T-Cells (activated) 10 4 CD19⁺ B-Cells (resting) <2 <2 CD19⁺ B-Cells(activated) <2 <2 CD14⁺ Monocytes (resting) <2 <2 CD14⁺ Monocytes(activated) <2 <2 One Color Staining PBLs (resting) <2 3 PBLs(activated) 50 27 CD4⁺ (resting) <2 <2 CD4⁺ (activated) 80 22 CD8⁺(resting) <2 <2 CD8⁺ (activated) 4 2 Granulocytes <2 <2 Platelets <2 <2Tumor Lines Molt-4, CEM, Hut 78, H9, Jurkat <2 <2 HPB-ALL, Sezary, T-AU<2 <2 Molt-3 20 3 B-LCL, Arent, RML, JY, KHY, PGf <2 <2 MSAB, CESS,9037, 9062 <2 <2 Dandi, Ramos, Namalwa <2 <2 Raji, NC37 30 4 U937,THP-1, HL-60 <2 <2 Kgla, K562, HEL <2 <2¹MCF = Mean Channel Fluorescence.²Cells indicated as “activated” had been stimulated with PHA for aboutthree days.

Example 3

Time Course of ACT-4-h-1 Expression Responsive to CD4⁺ T-cell Activation

CD4⁺ T-cells were tested for expression of ACT-4-h-1 receptors inresponse to various activating stimuli. CD4⁺ T-cells were purified fromperipheral blood mononuclear cells by solid-phase immunoadsorption(“panning”). 5×10⁴ CD4⁺ T-cells were cultured with an activating agentin microtiter wells containing RPMI medium supplemented with 10% humanserum. Three different activating agents were used: (1) 5×10⁴ irradiated(3000 rads) monocytes, (2) PHA (1 μg/ml) and (3) tetanus toxoid (5μg/ml). ³H-thymidine was added to the cultures 12-16 h before harvest.After harvest, cells were tested for the expression of cell surfaceantigens by incubation with various labelled antibodies (L106, anti-CD4and anti-CD8), as described by Engleman et al., J. Immunol.127:2124-2129 (1981).

FIG. 2 shows the appearance of ACT-4-h-1 in response to alloantigenactivation. Before activation, no expression was observed. Thepercentage of cells expressing the ACT-4-h-1 receptor increases withtime, peaking at about 30% after about seven days of alloantigenactivation. The results also show that essentially all cells expressingACT-4-h-1 also expressed the CD4 receptor and that essentially no suchcells expressed the CD8 receptor. FIG. 3 presents similar data for theappearance of ACT-4-h-1 in response to tetanus toxoid activation. Again,the percentage of cells expressing ACT-4-h-1 peaked at about seven days.However, at this time a higher percentage of cells (about 60%) expressedthe receptor. FIG. 4 presents similar data for the appearance ofACT-4-h-1 on CD4⁺ T-cells in response to PHA activation. In thissituation, the percentage of CD4⁺ T-cells expressing the receptor peaksat about 65% after three days of activation.

It is concluded that ACT-4-h-1 is a CD4⁺ T-cell activation antigen thatis expressed in response to diverse activating stimuli.

Example 4

Cloning ACT-4-h-1 cDNA

The cDNA clone for the ACT-4-h-1 receptor was isolated using a slightlymodified COS cell expression system, first developed by Aruffo & Seed,supra. RNA was isolated from 72-hour PHA activated human peripheralblood lymphocytes. Total RNA was extracted with TRI-reagent (MolecularResearch Center), and poly(A)+RNA was isolated by oligo dT-magnetic beadpurification (Promega). cDNA was synthesized by the method of Gubler &Hoffman, Gene 25:263-369 (1982) using superscript reverse transcriptase(Gibco/BRL) and an oligo dT primer. The blunted cDNA was ligated tonon-self-complementary BstXl adaptors and passed over a sephacryl S-400spin column to remove unligated adaptors and small fragments (<300 basepairs). The linkered cDNA was then ligated into a BstXl cut eukaryoticexpression vector, pcDNA-IRL, an ampicillin resistant version ofpcDNA-I(Invitrogen). The precipitated and washed products of theligation reaction were electroporated into E. coli strainWM1100(BioRad). Plating and counting of an aliquot of the transformedbacteria revealed a total count of 2 million independent clones in theunamplified library. Average insert size was determined to be 1.2 kb.The bulk of the library was amplified in liquid culture, 250 ml standardLB media. Plasmid was recovered by alkaline lysis and purified over anion-exchange column (Qiagen).

Sub-confluent COS-7 cells were transfected with the purified plasmid DNAby electroporation. Cells were plated on 100 mm dishes and allowed togrow for 48 hours. Cells were recovered from the plates with PBS-EDTAsolution, incubated with monoclonal antibody L106, and were pannedaccording to standard procedures. A second round panning revealedenrichment as numerous COS cells adsorbed to the plates. Episomal DNAwas recovered from the immunoselected cells by the Hirt method, andelectroporated into bacteria for amplification.

Bacteria transformed with plasmid from the second round Hirt preparationwere diluted into small pools of about 100 colonies. The pools wereamplified and their DNA purified and tested for the ability to conferexpression of the L106 antigen on COS-7 cells by immunofluorescence.Phycoerythrin-conjugated L106 antibody was used to stain COS-7 cellmonolayers and the cells were then examined by manual immunofluorescencemicroscopy. Miniprep DNA from four out of eight pools was positive whentested for expression. The pool with the best expression, pool E, wasdivided into smaller pools of ^(˜)12 colonies. Three out of eightsub-pools were positive, and sub-pool E1 was plated to allow for theanalysis of single colonies. Clone E1-27 was found to confer high levelexpression of ACT-4-h-1 receptor on the surface of transfected COScells.

Example 5

cDNA Sequence Analysis

The insert from the clone designated E1-27 was subcloned intopBluescript and sequenced by the dideoxy chain termination method, usingthe T7 polymerase autoread sequencing kit (Pharmacia) on an ALFsequencer (Pharmacia). Restriction mapping revealed several convenientsites for subcloning. Five subclones were generated in pBluescript andwere sequenced on both strands with M13 forward and universal primers.

The cDNA and deduced amino acid sequences of ACT-4-h-1 are shown in FIG.5. The ACT-4-h-1 cDNA sequence of 1,137 base pairs contains a 14-bp 5′untranslated region and a 209-bp 3′ untranslated region. An AATAAApolyadenylation signal is present at position 1,041 followed by an 80-bppoly A tail starting at position 1,057. The longest open reading framebegins with the first ATG at position 15 and ends with a TGA at position846. The predicted amino acid sequence is that of a typical type 1integral membrane protein. Hydrophobicity analysis reveals a putativesignal sequence following the initiating ATG, with a short stretch ofbasic residues followed by a longer stretch of hydrophobic residues. Apredicted signal peptide cleavage site is present at residue 22 or 24(the latter being the more likely by the criteria of von Heijne, NucleicAcids Res. 14, 4683-4690 (1986)) (incorporated by reference for allpurposes), leaving a mature protein of 253 amino acid residues (or 255amino acids, if cleavage occurs at the less probable site).Hydrophobicity analysis also reveals a single large stretch of 27hydrophobic residues predicted to be the transmembrane domain, whichpredicts an extracellular domain of 189 (or 191) amino acids and anintracellular domain of 37 amino acids. The extracellular domain iscysteine rich, where 18 cysteines are found within a stretch of 135amino acids. The predicted molecular mass (Mr) for the mature protein is27,400, and there are two potential N-glycosylation sites at amino acidresidues 146 and 160.

Comparison of the amino acid sequence of ACT-4-h-1 with known sequencesin the swiss-prot database using the BLAZE program reveals a sequencesimilarity with members of the nerve growth factor receptor superfamily.Amino acid sequences are at least 20% identical for NGF-R, TNF-R, CD40,41-BB, and fas/APO-1,. and 62% for OX-40, allowing for gaps anddeletions. Alignments of the various proteins reveal the conservation ofmultiple cysteine rich motifs. Three of these motifs are present inACT-4-h-1 and OX-40, compared with four such motifs in NGF-R and CD40.

Comparison of the nucleotide sequence of ACT-4-h-1 with known sequencesin the Genbank and EMBL databases using the programs BLAST and FASTDBrevealed a high degree of sequence similarity with only one member ofthe nerve growth factor receptor family, OX-40. Allowing for gaps andinsertions, the sequence identity is 66%. Comparison of the ACT-4-h-1and OX-40 nucleotide sequences reveals that both contain a 14-bp 5′untranslated region, and both contain approximately 80-bp poly A tails.In ACT-4-h-1, however, there is a slight lengthening of the 3′untranslated region from 187-bp to 209-bp, and there is a lengthening ofthe coding region from 816-bp to 834-bp, a difference of 18-bp or 6amino acid insertions. Aligning the two amino acid sequences revealsthat four of the amino acid insertions occur prior to the signalsequence cleavage site. Thus, the mature ACT-4-h-1 receptor proteincontains one more amino acid residue than OX-40 (i.e., 253 vs. 252 aminoacids). Remarkably, the ACT-4-h-1 nucleotide sequence is much more GCrich, than the OX-40 sequence (70% v. 55%) indicating that the twosequences will not hybridize under stringent conditions.

Example 6

Production of Stable ACT-4-h-1 Transfectants

An XbaI-HindIII fragment was excised from the construct described inExample 4, and inserted into XbaI/HindIII-digested pcDNA-I-neo(Invitrogen) to generate an expression vector termed ACT-4-h-1-neo (FIG.6). This vector was linearized with Sf1 and electroporated into threeeukaryotic cell lines. These cell lines were SP2/O (a mouse myelomaderived from the Balb/c strain), Jurkat (a transformed human T-cellline) and COS-7 (an adherent monkey cell line). After a 48-h recoveryperiod, transformed cells were selected in 1 mg/ml G418 (Gibco). Afterthree weeks of selection, neo-resistant cell lines were incubated with asaturating concentration of L106 antibody, washed and overlayered onto100 mm petri dishes coated with goat anti-mouse IgG to select for cellsexpressing ACT-4-h-1. After washing off unbound cells, adherent cellswere recovered and expanded in tissue culture. Cell lines were subjectto two further rounds of panning and expression. The resulting celllines were shown by direct immunofluorescence staining to expressabundant STAN-4-h-1 (FIG. 7).

Example 7

Production of an ACT-4-h-1-Immunoglobulin Fusion Protein

A soluble fusion protein has been constructed in which the extracellulardomain of ACT-4-h-1 is linked via its C-terminal to the N-terminal ofthe constant domain of a human immunoglobulin. The vector encodingACT-4-h-1 described in Example 4 was cleaved with SmaI and NotI toexcise all ACT-4-h-1 sequences downstream of the SmaI site including thetransmembrane, cytoplasmic and 3′ untranslated regions. The remainingregion encodes the soluble extracellular portion of ACT-4-h-1 (FIG. 8).The source of the immunoglobulin constant region to be joined to theACT-4-h-1 extracellular domain was a plasmid termed 5K-41BB-Eg1 (Proc.Natl. Acad. Sci. (USA) 89: 10360-10364) (incorporated by reference forall purposes). This plasmid contains a 1.3 kb BamHI/EagI genomicfragment encoding the hinge, CH2 and terminal CH3 domains of human Ig,isotype gamma 1. The fragment required modification for insertion intothe SmaI/NotI ends of the ACT-4-h-1 vector, while preserving the peptidereading frame across the SmaI junction to be formed by blunt-endligation. The vector 5k-41BB-Eg1 was cut with BamHI and the resulting 5′extensions were filled with Klenow fragment. The vector was then cutwith EagI releasing the 1.3 kb fragment with blunt and NotI compatibleends. This fragment was ligated with SmaI/NotI digested ACT-4-h-1vector. The ligation mix was electroporated into E. coli and multipletransformant clones screened with PCR using ACT-4-h-1 and IgG1nucleotide fragments as primers.

Plasmids containing the ACT-4-h-1-IgG1 coding were electroporated intoCOS cells. The cells were allowed to grow for five days at which pointtheir supernatants were harvested and sterile filtered through a 0.2micron membrane. The supernatants were tested for expression ofACT-4-h-1-IgG1 by dot blotting. Supernatants were blotted ontomitrocellulose and blocked with 5% nonfat dry milk. Replica blots wereprobed with antibody L106 or alkaline phosphatase-labelled goatanti-human immunoglobulin IgG (American Qualex). Antibody L106 wasdetected with an alkaline phosphatase labelled goat anti-mouse IgG.NBT/BCIP (Pierce) was used as a colorimetric substrate. High producingpositive clones were sequenced at the junction site to confirm propervector construction. The resulting fusion gene is depicted in FIG. 9.

For the purposes of clarity and understanding, the invention has beendescribed in these examples and the above disclosure in some detail. Itwill be apparent, however, that certain changes and modifications may bepracticed within the scope of the appended claims. All publications andpatent applications are hereby incorporated by reference for allpurposes to the same extent as if each were individually denoted asbeing incorporated by reference.

1. A purified ACT-4 receptor polypeptide that comprises at least fivecontiguous amino acids from an amino acid sequence shown in FIG.
 5. 2.The polypeptide of claim 1 that exhibits at least eighty percentsequence identity to the amino acid sequence of FIG.
 5. 3. Thepolypeptide of claim 2 having an antigenic determinant common to aprotein comprising the amino acid sequence shown in FIG.
 5. 4. Thepolypeptide of claim 3 that is a full-length polypeptide.
 5. Thepolypeptide of claim 4 comprising a domain selected from a group ofdomains consisting of a signal sequence, an intracellular domain, atransmembrane domain, and an extracellular domain.
 6. The polypeptide ofclaim 5 that comprises an extracellular domain.
 7. The polypeptide ofclaim 6, wherein said extracellular domain comprises an intrachain loopformed by disulfide bonding of two cysteine residues.
 8. The polypeptideof claim 7, wherein said extracellular domain comprises three intrachainloops, each formed by disulfide bonding of two cysteine residues.
 9. Thepolypeptide of claim 8, wherein said polypeptide is present on thesurface of activated CD4⁺ T-cells, and is substantially absent onactivated CD8⁺ T cells and resting T-cells.
 10. The polypeptide of claim9 that is naturally occurring.
 11. The polypeptide of claim 10 that ishuman.
 12. The polypeptide of claim 11 that has a molecular weight ofabout 50 kDa before deglycosylation and about 27 kDa thereafter.
 13. Thepolypeptide of claim 12 comprising the amino acid sequence of FIG. 5.14. An extracellular domain of a polypeptide of claim
 3. 15. Theextracellular domain of claim 14 comprising an intrachain loop formed bydisulfide bonding of two cysteine residues.
 16. The extracellular domainof claim 15 comprising three intrachain loops, each formed by disulfidebonding of two cysteine residues.
 17. The extracellular domain of claim16 that is soluble.
 18. The extracellular domain of claim 17 that iscapable of specifically binding to an ACT-4 ligand.
 19. Theextracellular domain of claim 18 that is immunogenic.
 20. Theextracellular domain of claim 19 that competes with an ACT-4-h-1receptor polypeptide for specific binding to an antibody.
 21. Theextracellular domain of claim 20 that is fused to a second polypeptide.22. The extracellular domain of claim 21, wherein the second polypeptideis a constant region of an immunoglobulin heavy chain.
 23. A polypeptideconsisting essentially of an epitope specifically bound by an antibodydesignated L106.
 24. An antibody that specifically binds to an ACT-4-h-1receptor polypeptide.
 25. The antibody of claim 24 that is a monoclonalantibody.
 26. The antibody of claim 25 that inhibits activation of CD4⁺T-cells.
 27. The monoclonal antibody of claim 25 that stimulatesactivation of CD4⁺ T-cells.
 28. The antibody of claim 25 that competeswith an antibody designated L106 for specific binding to an ACT-4-h-1receptor polypeptide.
 29. The antibody of claim 25 that competes with anantibody designated L106 for specific binding to activated CD4⁺ T-cells.30. The antibody of claim 25 that is fused to a coat protein of afilamentous phage.
 31. The antibody of claim 29 that is L106.
 32. Ahumanized antibody comprising a humanized heavy chain and a humanizedlight chain: (1) the humanized light chain comprising threecomplementarity determining regions (CDR1, CDR2 and CDR3) having aminoacid sequences from the corresponding complementarity determiningregions of a L106 antibody light chain, and having a variable regionframework sequence substantially identical to a human light chainvariable region framework sequence; and (2) the humanized heavy chaincomprising three complementarity determining regions (CDR1, CDR2 andCDR3) having amino acid sequences from the corresponding complementaritydetermining regions of a L106 antibody heavy chain, and having avariable region framework sequence substantially identical to a humanheavy chain variable region framework sequence; wherein the humanizedantibody specifically binds to an ACT-4-h-1 receptor polypeptide with abinding affinity that is within three-fold of the binding affinity of aL106 antibody.
 33. An immunotoxin comprising the antibody of claim 31fused to a toxin polypeptide.
 34. The antibody of claim 25 thatspecifically binds to a different epitope on an ACT-4-h-1 receptorpolypeptide than that specifically bound by an L106 antibody.
 35. Afragment of the antibody of claim 31 that specifically binds to anACT-4-h-1 receptor polypeptide.
 36. A hybridoma producing antibody L106,said hybridoma deposited as ATCC______.
 37. A method of screening anantibody for specific binding to the same epitope as that bound by anL106 antibody, said method comprising: providing a solution comprisingan antibody to be screened, said L106 antibody, and an ACT-4-h-1receptor polypeptide, said polypeptide specifically binding to said L106antibody; and measuring specific binding between said polypeptide andsaid L106 antibody, or between said polypeptide and said antibody to bescreened, to indicate whether said antibody to be screened reacts withthe same epitope as said L106 antibody.
 38. A method of localizing anepitope specifically bound by an L106 antibody, said method comprising:providing a family of ACT-4-h-1 receptor polypeptides, each member ofsaid family comprising a contiguous segment of at least four aminoacids; and measuring specific binding between a polypeptide from saidfamily and said L106 antibody to indicate the presence of said epitopewithin said polypeptide.
 39. A nucleic acid fragment encoding a heavychain of an antibody of claim
 31. 40. A nucleic acid fragment encoding alight chain of an antibody of claim
 31. 41. A nucleic acid fragmentencoding an ACT-4 polypeptide of claim
 1. 42. The nucleic acid fragmentof claim 41 that exhibits at least eighty percent sequence identity withthe nucleic acid sequence shown in FIG.
 5. 43. The nucleic acid fragmentof claim 42 that encodes a full-length ACT-4 polypeptide.
 44. Thenucleic acid fragment of claim 43 comprising the translated region ofthe nucleic acid sequence shown in FIG.
 5. 45. An isolated cell linecontaining a nucleic acid fragment of claim
 41. 46. The isolated cellline of claim 45, wherein the ACT-4 receptor polypeptide is expressed onthe surface of said cell.
 47. The isolated cell line of claim 46 that isstable.
 48. The isolated cell line of claim 47, wherein said nucleicacid fragment is incorporated in the genome of said cell line.
 49. Theisolated cell line of claim 48, wherein said cell line is COS-7.
 50. Amethod of screening for immunosuppressive agents, said methodcomprising: contacting an ACT-4-h-1 receptor polypeptide with apotential immunosuppressive agent; and detecting specific bindingbetween said ACT-4-h-1 receptor polypeptide and said agent, saidspecific binding indicative of immunosuppressive activity.
 51. Themethod of claim 50, wherein said ACT-4 receptor polypeptide isimmobilized to a solid surface.
 52. A method for screening for an ACT-4ligand, said method comprising: contacting a biological samplecontaining said ACT-4 ligand with an ACT-4-h-1 receptor polypeptide;isolating a complex formed between said ligand and said ACT-4-h-1receptor polypeptide; and dissociating said complex to obtain saidligand.
 53. A method of suppressing an immune response in a patientsuffering from an immune disease or condition, said method comprisingadministering to a patient a therapeutically effective dose of apharmaceutical composition comprising a pharmaceutically active carrierand a monoclonal antibody of claim
 26. 54. A method of inducing animmune response to a selected antigen comprising: administering amonoclonal antibody of claim 27 to a patient; and exposing said patientto said selected antigen.
 55. A method of detecting activated CD4⁺T-cells, said method comprising: contacting a tissue sample from apatient with a monoclonal antibody of claim 24 and detecting specificbinding between said monoclonal antibody and said tissue sample toreveal the presence of said activated CD4⁺ T-cells.
 56. The method ofclaim 55, wherein the presence of activated T-cells revealed by saidspecific binding is diagnostic of a disease or condition of the immunesystem.
 57. A pharmaceutical composition comprising a monoclonalantibody of claim 26 and a pharmaceutically acceptable excipient.
 58. AnACT-4 ligand that specifically binds to an ACT-4-h-1 receptorpolypeptide.